In order to recover a desired polymer contained in a latex from a polymer latex prepared by emulsion polymerization or suspension polymerization, granulating processes for coagulating and granulating the latex are required. Polymers are recovered from polymer lattices by the following procedure: a coagulant is added to a polymer latex at a temperature sufficiently lower than the softening temperature of the polymer to form coagulated latex particles. The resulting mixture is then heated to at least the softening temperature of the polymer to produce slurry, followed by dehydrating and drying. Thus, a powdered polymer is recovered. In this process, the reason for setting a temperature sufficiently lower than the softening temperature of the polymer when a coagulant is added to a polymer latex is to suppress the secondary coagulation among the coagulated latex particles generated. In general, when a coagulant is added at a temperature more than the softening temperature of the polymer, generation of rough and large coagulated latex particles frequently occurs, and in the worst case, it may happen that the whole latex particles get aggregated.
As opposed to the above process, granulating processes of a polymer latex which can obtain coagulated latex particles having satisfactory powder properties, a gas-phase coagulation process (for example, see JP-A-53-30647), a moderate coagulation process (for example, see JP-A-60-217224), a granulating process using a spray dryer, and the like are widely known. Even though these processes are employed, in view of suppressing the secondary coagulation, granulating processes at a temperature lower than a softening temperature of a polymer is desirable, and for the purpose, in general, the granulating processes are conducted at a temperature around or lower than the softening temperature of the polymer.
However, it can not be better to set a temperature lower in the above granulating processes, when the granulation temperature is set too lower than the softening temperature of the polymer, mechanical strength of the coagulated latex particles generated tends to lower, thus, a large amount of a fine powder is generated, which may cause a factor of problems in the steps such as a filter fabric clogging.
Namely, in the conventional granulating processes, it is important to suppress the secondary coagulation and generation of fine powder in order to obtain coagulated latex particles having a desired particle size with a satisfactory yield, and it is essential to operate in a temperature range for granulating at which influences of both secondary coagulation and generation of fine powder is minimum (generally, from around a polymer softening temperature to about 10° C. lower than it). However, when the granulating temperature fluctuates due to various factors during producing process, it is a problem of producing process to cause troubles such as lowering the yield by secondary coagulation and impairing filterability by generation of fine powder.
In addition, the conventional granulating processes has restriction in compositions of a polymer possible to be recovered, as for a rubbery polymer latex having the softening point of the polymer at most 0° C., when water is a medium, it is difficult even to set a temperature of the system within the range of the optimum granulating temperatures. Also when the granulating processes are conducted at around 0° C., the secondary coagulation frequently occurs, and the coagulated latex particles can not be obtained with a satisfactory yield.
In addition to the above granulating techniques, as a process for granulating a rubbery polymer latex having a softening temperature of the polymer of room temperature or lower, which is extremely difficult to be granulated due to easiness in the secondary coagulation, a process of adding a high-molecular weight polyanion having a carboxyl group and/or a hydroxyl group in its molecule to a rubber latex, and dropping the mixed latex into an aqueous solution containing at least one alkaline earth metal is known (for example, see JP-A-52-37987).
In this process, however, for example, at least 2 to 8 parts by weight and preferably 4 to 6 parts by weigh of the high-molecular weight polyanion must be added relative to 100 parts by weight of rubber solid content of the rubber latex, the viscosity of the resulting mixed latex must be adjusted to 200 to 8,000 m·Pa·s, and subsequently the latex must be dropped from 1 to 80 cm higher than the liquid level of a coagulant. Thus, according to the description of this process, satisfactory spherical coagulated latex particles cannot be produced unless many conditions are satisfied.
In general, it is easily assumed that the addition of 2 parts by weight or more of a high-molecular weight polyanion to a polymer latex causes the following problems, and thus this is not a satisfactory process. Examples of the problems are as follows: (1) The original quality (for example, thermal stability) of a recovered polymer itself used for various purposes may be deteriorated; (2) The addition of a large amount of high-molecular weight polyanion leads to the significant increase in the production cost; and (3) Since the viscosity of the latex, which is generally 10 m·Pa·s or less, must be adjusted to 200 m·Pa·s or more and preferably 1,000 m·Pa·s or more by adding the high-molecular weight polyanion, the transferring property of the resulting latex liquid is impaired.
On the other hand, we developed the technology which can recover the desired coagulated latex particles with a satisfactory yield, suppressing the secondary coagulation and generation of a fine powder, under the temperature condition having as a broad range as possible, without deteriorating the original quality of the polymer itself, and filed the technology (Japanese Patent Application No. 2005-052783). In this process, the coagulated latex particles having a desired particle size can be recovered with an extremely satisfactory yield compared with the conventional granulating processes, by suppressing the secondary coagulation among coagulated latex particles entering into the aqueous phase. However, in this process, it is difficult to suppress the secondary coagulation by collision and uniting among the coagulated latex particles in the gas phase, thus, further development in efficiency has been desired.